The present invention relates to the field of linear variable differential transformers or "LVDTs". More specifically, it relates to an improvement in the configuration of the secondary coils in LVDTs, and to the novel process by which the improved secondary coils are made.
As is well known, LVDTs are, basically, position sensors. They operate on the principle that a ferromagnetic core is moved through the interior of a hollow bobbin onto which a single primary coil and at least two secondary coils of wire are wound. The magnitude of the magnetic coupling of the coils to one another depends on the position of the movable core. Voltage and phase shift measurements, which may be obtained by comparing the outputs of the two secondary coils with the input to the primary coil, are indicative of the position of the movable core.
A troublesome problem with prior art LVDTs has been the achievement of good output linearity. Improved results in this respect have been provided by varying the pitch of the secondary coils along the length of the bobbin. In such an arrangement, a first secondary coil is wound over the primary coil with its pitch increasing in one direction, and a second secondary coil is wound over the first secondary coil, with its pitch increasing in the opposite direction. The three coils are thus arranged concentrically, with the primary coil innermost, the first secondary coil in the middle, and the second secondary coil outermost.
The above-described "concentric secondary" arrangement has some disadvantages, however, as a result of the two secondary coils not being equidistant from the primary coil. For example, there are inherent imbalances between the two secondaries in DC resistance, secondary impedance, and output voltage, all resulting from unequal inductive coupling between the primary coil and each of the two secondary coils. These imbalances can be corrected, to some extent, by careful adjustment of the pitch variations in the two secondary coils, but such adjustment complicates the manufacturing process. In addition, the concentric secondary arrangement results in a small, but measurable, intercoil capacitance which can adversely affect the accuracy of the measurements obtained with the LVDT.
One approach to solving the above-mentioned problems is suggested in U.S. Pat. No. 3,017,589 to Chass. The Chass patent shows an LVDT having a primary coil, a first secondary coil assembly and a second secondary coil assembly. Each of the secondary coil assemblies comprises a plurality of discrete secondary coil units. The coil units of the first secondary coil assembly are wound over the coil units of the second secondary coil assembly along one half of the length of the bobbin, and the coil units of the second secondary coil assembly are wound over the coil units of the first secondary coil assembly along the other half of the bobbin. This arrangement equalizes the average distance between each secondary coil assembly and the primary coil, thereby minimizing imbalances between the two secondary coils. In addition, the symmetrical arrangement of the two secondaries minimizes the deleterious effects of intercoil capacitance.
One important shortcoming of the device described in the Chass '589 patent is its requirement of a plurality of discrete secondary coil units, each of which must be separately wound and terminated. This arrangement thus increases the complexity and cost of manufacture. In addition, the need for electrical connections among all of the secondary coil units can result in reduced durability and reliability for the LVDT.
Thus, the need has been felt to provide the advantages of the Chass '589 device without the disadvantages of a multiplicity of discrete secondary coil units. Such a need could be fulfilled by an LVDT which employs two continuous, unitary secondary coils, each occupying a major portion of the bobbin's length, but crossing over one another near the midpoint of the bobbin. Until the advent of the present invention, however, there were no practical means available for winding a linearized and balanced pair of secondary coils in this manner.